Geek Answers: Why does NASA use so much gold foil?

I can go down to the local grocery store and buy aluminum foil for roughly 40 cents per square foot, and NASA’s complaining about their budget? Why would the nation’s space agency have to opt for make their foil out of gold rather than reliable old aluminum, or copper, or even silver if they’ve got to get fancy about it? I mean what’s next, a space suit made of carbon fiber?

The fact is that NASA cuts corners wherever it can, but when soaring beyond the Earth’s skies there are precious few areas that can tolerate less than the best. While most people think of gold as a precious metal and as a building material embraced by people with more money than sense, it is also one of the heaviest, most uniquely workable metals that isn’t dangerously radioactive.

When you see a space contraption draped in gold foil, remember that the foil is probably a heat shield or, more practically, a radiation shield. The sun transmits heat on Earth mostly by warming the atmosphere, and we experience that heat by convection, like a turkey in the oven. In space direct impact from radiation transfers heat, like a dish warmed in a microwave. As a result, keeping instrumentation cold is less about insulation than about reflection, and gold has some very desirable qualities in this regard.

Four metal compared for their ability to reflect different wavelengths of light.

As we can see in the figure to the right, gold reflects infrared radiation (above roughly .7 µm) as well as any of our candidate metals, which is a major part of keeping tech-heating rays out of our hair. However, it also reflects as much or more UV radiation (roughly .35 µm) than its competitors while absorbing quite a bit of visible light. This means that it won’t create blinding reflective hotspots for astronauts, and its heavy atomic weight lets it soak up quite a bit of that visible light before heating to any harmful extent.

Gold also does not rust or tarnish in air the way copper or silver do, meaning it requires less care and maintenance to keep mission-ready, and it remains softer and more malleable than aluminum when stretched. Anyone who has ever tried to unroll and re-use a piece of aluminum foil in the kitchen knows how unwilling it is to forgive even the slightest crease. All metal foils have this property to an extent, but gold foil does present a slightly easier workflow than its cheaper competitors.

A gold-coated collector collector

Gold is used by NASA in all kinds of contexts. It’s used in external reflectors like those seen in these photos, but it’s also found in astronauts’ visors, filtering out IR radiation to protect astronauts’ eyes. When coupled with an ultra-violet filter like polycarbonate, this makes a shield for both infra-red and ultra-violet radiation while letting a good amount of visible light through to the astronaut.

Gold is also used in many collectors. Its state is assayed before and after a mission to determine what sorts of particles hit it while exposed to space. Even micro-electronics are often made from gold, since the element is an excellent conductor while resisting corrosion and the buildup of static electricity. Even mirrors incorporate gold at the atomic level, with laser-deposited layers or gold providing that extra bit of infrared reflectance needed to create ultra-high resolution images.

So don’t think NASA’s use of gold is wasteful. Whether it’s a gold-coated sheet of mylar or an atomically thin spray of gold on a space mirror, the element has a combination of properties like no other. Until materials science can outstrip gold’s remarkable confluence of reflectance, durability, conductivity, and physical workability, it will continue to have a shining, opulent place in human excursions into space.